What is the key principle behind monitored natural attenuation (MNA) as a groundwater remediation strategy?

The key principle behind monitored natural attenuation (MNA) as a groundwater remediation strategy is to rely on naturally occurring physical, chemical, and biological processes to reduce the mass, toxicity, mobility, volume, or concentration of contaminants in groundwater over time. These natural processes include biodegradation (the breakdown of contaminants by microorganisms), dispersion (spreading of contaminants due to groundwater flow), dilution (reduction of contaminant concentration by mixing with uncontaminated water), sorption (binding of contaminants to soil particles), volatilization (transfer of contaminants from groundwater to soil gas), and chemical reactions (e.g., oxidation-reduction). MNA is not a "do-nothing" approach; it requires rigorous site characterization to demonstrate that these natural processes are actively occurring and are capable of achieving remediation goals within a reasonable timeframe. Regular monitoring of groundwater quality is essential to track the progress of attenuation and ensure that the plume is shrinking and not posing an unacceptable risk to downgradient receptors. For example, if a plume of benzene is detected in groundwater, MNA might be considered if evidence shows that naturally occurring bacteria are degrading the benzene, and the plume is stable or shrinking.